1. Field of the Invention
This invention relates to a pyrimidine derivative as a novel liquid crystal compound used as an electrooptical display material. More particularly it relates to a compound which is suitable as a component of a liquid crystal composition useful for low voltage drive, and a liquid crystal composition containing the liquid crystal compound and having superior characteristics.
2. Description of the Related Art
Various characteristics are required for liquid crystal materials used for liquid crystal display devices, depending upon display modes, use applications, etc., and when twisted nematic liquid crystal materials which have now been most broadly utilized are taken as an example, the following characteristics are required:
1. colorless, stable to heat and light, and stable electrically and chemically; PA1 2. broad service temperature range; PA1 3. high electrooptical response rate; PA1 4. low drive voltage; PA1 5. steep rise of the voltage-light transmittance characteristic and small temperature dependency; and PA1 6. broad range of view angle.
Among these various characteristics, the low drive voltage and the broad service temperature range are particularly important. In the case of the twisted, nematic liquid crystal materials, there is the following relationship between the threshold voltage value Vth and the dielectric anisotropy value .DELTA..epsilon.: ##EQU1## wherein K.sub.11, K.sub.22 and K.sub.33 represent the elastic constants relative to splay, twist and bend, respectively, and K represents a constant of proportionality. Thus, in order to lower the threshold voltage, it is necessary that the .DELTA..epsilon. value be positive and large.
As for liquid crystal compounds having a large .DELTA..epsilon. value heretofore used, 4-alkylbenzoic acid-4-cyanophenyl esters, 4-alkyl-4'-cyanobiphenyls, etc. are exemplified. These compounds have a low N-I point, so the nematic mesomorphic range of compositions using these compounds is narrowed. Further, 4-alkyl-4"-cyanoterphenyls, 4-(trans-4'-alkylcyclohexyl)-4"-cyanobiphenyls, etc. have a high N-I point, but their elastic constant is so large that the threshold voltage is elevated. Further, as a pyrimizine derivative, a compound expressed by the formula ##STR2## is disclosed in U.S. Pat. No. 4,640,795. This compound has a relatively large .DELTA..epsilon. value, but it exhibits no liquid crystal phase; hence it has a drawback of lowering the clearing point of liquid crystal compositions using this compound. Furthermore, U.S. Pat. No. 4,752,414 discloses a compound expressed by the formula ##STR3## but the compound has a .DELTA..epsilon. value as small as about +7, and its compatibility with other liquid crystal compounds at low temperatures is inferior; hence it is not useful as a component of liquid crystal compositions for low voltage drive.